The development of the bone marrow cavity is an coordinated process in which hematopoietic stem cell (HSC) migrate and colonize spaces carved out of embryonic bone and cartilage. Thus, an intimate physical association between bone cells (osteoblasts or OBs) and HSCs is established early in life. The focus of these studies relate to whether this spatial relationship between bone and blood also mirrors a functional interdependence between the two tissues. We hypothesize that osteogenesis and hematopoiesis are interrelated and interdependent developmental processes regulated by direct cell- to-cell interaction. The overall goal of this proposal is to define the specific mechanisms and consequences of HSC-OB interactions in normal as well as in abnormal hematopoiesis. Based upon our preliminary data we will delineate if HSC-derived cytokines establish a paracrine loop with OB intermediaries, which in turn release cytokines necessary for HSC survival (Aim 1A). We will determine whether HSCs themselves alter the cytokines produced by OBs in response to hematopoietic stresses (Aim 1B). Such perturbations may arise in response to functional demands that affect OB cytokine production or that influence OB maturation. Furthermore, we have also learned that sialated-N-linked glycoproteins mediate the initial tethering of HSCs to OBs and LFA-1 and VLA-4 receptors on HSCs provide firm cell-cell contact thereby assuring the survival of HSCs on OBs. In the second aim monoclonal antibodies to candidate receptor proteins will be made to further characterize the sialated N-linked glycoproteins responsible for the initial tethering of HSCs to OBs (Aim 2A). In the second part of this aim, we will determine of OBs respond to HSCs by altering the expression of their adhesion molecules, and if soluble recombinant receptor ligands to VLA-4 and LFA-1 will substitute for direct contact between HSCs and OBs to ensure HSC survival in vitro (Aim 2A). In the final aim, we will determine if OBs are required for survival of HSCs within the bone marrow of living animals using transgenic mice which allow us to induce the death of mature OBs (Aim 3A), and if OBs at different stages of maturation support HSC survival (Aim 3B). The results of these investigations will provide important new insights into the nature of the interactive biology of blood and bone in health and disease. These investigations will ultimately lead to new strategies for the treatment of hematopoietic and osseous disorders.